Short answers, pulled from the story.
A monarchy is a hereditary form of government in which political power is legally passed to the family members of a monarch, a head of state who rules for life. Monarchs usually gain power through succession laws, though some gain authority by election. The two conventional types are absolute monarchy and constitutional monarchy.
Forty-three sovereign nations have a monarch as head of state today. This includes fifteen Commonwealth realms that share King Charles III as their head of state. Roughly twelve of the world's monarchies are absolute monarchies governed as autocracies.
In an absolute monarchy the monarch rules with absolute power over the state and government. In a constitutional monarchy the monarch's power is subject to a constitution and is usually a ceremonial figurehead with limited or no political power. Semi-constitutional monarchies fall between the two, where the monarch rules under a constitution but keeps substantial powers.
Japan is the world's oldest existing monarchy, having had an emperor according to legend since Emperor Jimmu, who reigned from 660 to 585 BCE. Among the oldest recorded and evidenced monarchies were those of Narmer, Pharaoh of Ancient Egypt around 3100 BCE, and Enmebaragesi, a Sumerian King of Kish around 2600 BCE.
Four elective monarchies exist today: Cambodia, Malaysia, and the United Arab Emirates, which are 20th-century creations, and the papacy, which is ancient. In an elective monarchy there is no popular vote; a small electoral body chooses the ruler, who then reigns for life or a defined term.
In 1980 Sweden became the first monarchy to declare full cognatic primogeniture, letting the eldest child of either sex ascend the throne. The Netherlands followed in 1983, Norway in 1990, Belgium in 1991, Denmark in 2009, and Luxembourg in 2011. The United Kingdom adopted equal primogeniture on the 25th of April 2013.